Nearly two-thirds of the new Iraqi army units are only "partially capable" of fighting rebels - and only then with US help, while up to half of Iraq's new police forces cannot carry out their duties, according to a Pentagon assessment reported in the New York Times .

The article claims that only "a small number" of Iraqi security forces are able to fight insurgents alone.

The assessment, which emerged when a recently declassified document was given to the newspaper by a Senate staff employee, said one third of the army is capable of "planning, executing and sustaining counterinsurgency operations" but only with support from US and allied forces.

The Pentagon report is mentioned in a written response that General Peter Pace, the incoming chairman of the joint chiefs of staff provided to the Senate armed services committee. It has not been publicly released.

At his confirmation hearing on June 29, the Senate instructed Gen Pace to give an unclassified account of the Iraqi's abilities to maintain law and order.

In his seven-sentence response, he said: "The majority of Iraqi security forces are engaged in operations against the insurgency with varying degrees of cooperation and support from coalition forces."

About half of the police forces are still in the stages of being set up and "are not yet capable of conducting operations", he wrote.

The New York Times claims this is the first time Iraq's security abilities have been described in detail. US defence officials have so far been reluctant to give the rebels any "advantage" by discussing their capabilities.

"It's not for us to tell the other side, the enemy, the terrorists, that this Iraqi unit has this capability and that Iraqi unit has this capability," the US defence secretary, Donald Rumsfeld, said.

The article has raised concern over how long US troops are now expected to remain in Iraq. US defence officials have always maintained that the 160,000 US-led troops will not withdraw from Iraq until the country's own security forces are able to keep order.

Today's report in the New York Times confirms an eyewitness account by Guardian writer Ghaith Abdul-Ahad that the Iraqi army is struggling against a ruthless insurgency because of poor equipment and rampant sectarianism.

Abdul-Ahad discovered deep divisions between Iraqi and US forces, and that units often work with no maps, inadequate vehicles and helmets with no cooling insulation - they feel like "pouring melted iron on our heads".